The real power of comm comes with the OPTIONS not explained above. The comm command can exclude columns of the output using -1, -2, or -3 where the numbers correspond to the columns described above. So to see only the lines that are in file1 but not in file2, you would run comm -23 file1 file2. Similarly to see only the lines that are in file2 but not file1, run comm -13 file1 file2. And finally, to see only the files common to both files, run comm -12 file1 file2.